Variable transformer



Aug. 25, 1959 s, PARKER 2,901,715

VARIABLE TRANSFORMER Original Filed April 30, 1952 Fig. 2

INVENTOR. SAM E. PARKER ATTORNEYS United States Patent VARIABLETRANSFORMER Sam E. Parker, San Diego, Calif.

Original application April 30, 1952, Serial No. 285,298, now Patent No.2,762,971, dated September 11, 1956. gigidetsl and this applicationApril 12, 1956, Serial No.

1 Claim. (Cl. 33687) (Granted under Title 35, US. Code (1952), see. 266)This is a divisional application of application Serial No. 285,298,filed April 30, 1952, and issued as Patent No. 2,762,971, on September11, 1956.

This invention relates to a variable transformer and more particularlyto a variable RF transformer utilizing a shielded rotatable secondarywinding.

In the prior art there has long existed the problem of providingadequate magnetic shielding between primary and secondary of RF couplingtransformers. This has been particularly true of the type in which thesecondary winding is rotatably disposed within the primary winding. Theprior art shielding consisted of encasing the primary winding with amagnetic shield. The main disadvantages of this prior art expedient layin the generation of excessive eddy currents, and the inequal andasymmetrical capacity between the secondary and ground for variouscoupling positions.

It is thus an object of the present invention to provide a shielded RFvariable coupling transformer in which the ground capacity effects areconstant for any coupling position.

A further object is the provision of an inter-winding transformer shieldwith a minimum of eddy currents.

According to the invention a secondary winding is insulatedly mountedwithin a primary Winding on a rotatable shaft. A magnetic shield whichis divided into two symmetrical parts resembling two cup-shaped halvesof a closed right circular cylinder is mounted on the shaft with theopen ends adjacent one another. The cylinder thus substantiallysurrounds the inner coil or secondary winding. One of the two halves isconductively connected to the shaft, which is grounded. The two parts ofthe shield are connected to one another by a single conductor at onepoint only on each shield, minimizing eddy currents in the shield. Sincethe shield rotates with the inner coil or secondary, capacity to groundwill remain constant regardless of the position of the shaft.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same becomes better understood byreference to the following detailed description when considered inconnection with the accompanying drawings wherein:

Fig. l is a sectional view of a preferred embodiment of the invention;and

Fig. 2 is a prospective view of the shielding of Fig. 1.

Referring now to the drawings, there is shown in Fig. 1 a preferredembodiment of the present invention. The secondary winding 80 is rigidlysecured to shaft 82 and positioned inside stationary primary winding 84which is maintained in place by bracket 85. Connections to the primaryare through leads 81 and 83. A slotted shield 86 surrounding thesecondary is symmetrical to ground at all times so capacitive couplingto ground is constant, capacitive coupling between the two windings iseliminated, and single-point injection of the induced volt- 2,901,715Patented Aug. 25, 1959 age is provided. The slots in shield 86 areeffective to cut down on losses due to eddy curents. The degree ofcoupling (M) of the two windings is indicated by the setting of pointer88 on scale 90 as secondary is rotated by means of knob 92. One end ofsecondary winding 80 is connected to the coupling circuit by means oflead 98 which is insulated from shaft 82 and shielding 86. From lead 98,the current flows through conductor 102, which is a concentric sleeveaffixed to said shaft and insulated therefrom, and is taken off by meansof wiper arm 104 as the shaft rotates. The other end of secondarywinding 80 is simply connected through lead 103 to shaft 82 which inturn is electrically connected to shielding 86 and to grounded bushing105. The two sections of shielding 86 are connected through two opposingfingers at point 106.

Fig. 2 shows, in a cutaway perspective view, one section of shielding86. Eddy current losses are held to a very low value by making the slotscontinuous almost to the hub and by forming the shielding in twogenerally similar sections. In practice, both sections of shielding 86may be grounded by connecting one finger of the insulated section withan adjacent finger of the grounded section. The single point of contactdoes not materially reduce the efficiency of the shielding.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claim the invention maybe practiced otherwise than as specifically described.

What is claimed is:

A variable transformer comprising a first hollow cylindrical form, afirst coil wound on said first form, a second hollow cylindrical formdisposed within said first form, a second coil wound on said secondform, a shaft rotatably mounted in said first form normal to thelongitudinal axis thereof, said 2nd form mounted on said shaft with thelongitudinal axis thereof normal thereto, a sleeve of insulationsurrounding said shaft and extending from one side of said second formthrough said first form, a conductive sleeve surrounding said insulationsleeve, 21 contact brush mounted outside said first form in slidingelectrical contact with said conductive sleeve, an electrostaticcylindrical shield symmetrically disposed around said second coil andwithin said first form, said shield comprising a first part conductivelymounted on said shaft between one side of said 2nd coil and said firstform, and a second part insulatedly mounted on said shaft between theother side of said 2nd coil and said first form, said shield beingconcentric and rotatable with said shaft, and said first and secondparts comprising a bisected closed right circular cylinder, the plane ofbisection of said right circular cylinder being normal to thelongitudinal axis thereof, a plurality of slots in said shield parallelto said shaft, said slots having extensions forming radial slots in theends of said shield, one end of said second coil conductively connectedto said shaft and the other end of said second coil conductivelyconnected to said conductive sleeve and a single conductor electricallyconnecting said two parts of said shield for minimizing eddy currentstherein.

References Cited in the file of this patent UNITED STATES PATENTS1,422,429 Filkins July 11, 1922 1,500,827 Langley July 8, 1924 1,624,473Clement Apr. 12, 1927 1,651,658 Young Dec. 6, 1927 2,138,209 Seeley Nov.29, 1938

